1. Field of the Invention
The present invention relates to a dielectric ceramic composition which is advantageously used in a laminated ceramic capacitor having an internal electrode formed of a base metal such as nickel or nickel alloy. The present invention also relates to a laminated ceramic capacitor which is formed from the dielectric ceramic composition and to a method for producing the capacitor.
2. Description of the Related Art
A laminated ceramic capacitor includes a laminate formed of a plurality of laminated dielectric ceramic layers and an internal electrode laminated therein. Recently, the internal electrode has been formed of an inexpensive base metal such as Ni rather than an expensive noble metal such as Ag or Pd in order to reduce cost.
When the internal electrode is formed of a base metal such as Ni, the electrode must be fired in a reducing atmosphere so as to avoid oxidizing the base metal. However, when fired in a reducing atmosphere, a ceramic formed of barium titanate is disadvantageously reduced to become semiconductive.
In order to solve this problem, there has been developed a technique for preventing reduction of dielectric materials by modifying the ratio of the barium sites/titanium sites in the barium titanate solid solution such that it exceeds the stoichiometric ratio (Japanese Patent Publication (kokoku) No. 57-42588). Through this technique, a laminated ceramic capacitor having an internal electrode formed of a base metal such as Ni can be put into practical use, and production of such capacitors has increased.
With recent advances in development of electronics, miniaturization of laminated ceramic electronic elements has progressed rapidly. In the field of laminated ceramic capacitors, trends towards miniaturization and increased capacitance are also noticeable. In addition, laminated capacitors must have an electrostatic capacity that is higher and have a lower dependence on temperature. Thus, a variety of materials having high dielectric constant and excellent temperature-related characteristics have been proposed and put into practical use.
Thus far, all the proposed materials comprise BaTiO.sub.3 as a primary component and a rare earth element, which is diffused into BaTiO.sub.3 grains during sintering, as an additive. Grains that constitute the obtained sintered compacts are known to have a core-shell structure comprising a core portion containing no diffused additive component and a shell portion containing the diffused additive component. Therefore, the combination of the core portion and the shell portion--which differ according to the temperature dependence of the dielectric constant--provides a composition whose dielectric constant has a low dependence on temperature.
These materials realize laminated ceramic capacitors having high electrostatic capacity and low dependence on temperature, and thus have greatly contributed toward broadening of the market.
However, the core-shell structure, which is attained through sintering of ceramics and control of diffusion of the additive component, also involves a disadvantage. Specifically, as sintering progresses the additive component diffuses excessively to fail to provide low dependence on temperature, whereas insufficient sintering results in poor reliability. Achieving control of sintering and diffusion is relatively difficult with the above-described materials, causing undesirable variation in the temperature dependence of dielectric constant.
Furthermore, in order to satisfy demand for miniaturization and high electrostatic capacity, dielectric ceramic layers formed in a laminated compact must be made thinner and the laminates must comprise a greater number of layers. However, when the ceramic layers become thin, a smaller number of ceramic grains are included between internal electrodes and this remarkably deteriorates the reliability of the capacitor. Thus, the decrease in the thickness must be limited. Therefore, development of materials having high reliability and exhibiting low variation in dielectric constant with temperature and electric field must be achieved through a decrease in the size of ceramic grains.
Meanwhile, many electronic elements such as those used in automobiles are used in a high-temperature environment, and therefore those whose characteristics remain stable at high temperature are desired. Specifically, there is desired a laminated ceramic capacitor of high reliability and having a dielectric constant having a low temperature dependence at higher temperature (e.g., 150.degree. C.).
However, the sinterability of conventional materials having a core-shell structure and diffusion of an additive component increases as the BaTiO.sub.3 grains become smaller, which causes difficulty in maintaining low temperature dependence characteristics. Since BaTiO.sub.3 exhibits a large variation in dielectric constant at high temperature (e.g., 150.degree. C.), maintaining a dielectric constant having low temperature dependence up to high temperature is relatively difficult.
As described hereinabove, according to the state of the art, realization of a sufficiently thin laminated ceramic capacitor and a dielectric constant of sufficiently low temperature dependence by use of a material having a core-shell structure is difficult.